The GI calculation doesn't account for the potential "rebound" hypoglycemia, which can indeed make a food's glycemic impact appear better on paper than it might be in practice.
Glycemic responses can vary significantly between individuals and even for the same individual on different days. Factors like stress, sleep, previous meals, and overall health status can all influence how a person responds to a particular food.
The GI focuses solely on glucose response and doesn't consider fructose levels in the blood. This can be particularly relevant for fruits and foods containing high-fructose corn syrup.
The GI doesn't take into account realistic portion sizes, which can lead to misleading conclusions about a food's actual impact on blood sugar in typical consumption patterns - Carrots have a high glycaemic index – if you consume 50 grams of carbohydrate in carrots, your blood sugar will rise pretty high. But you would have to eat 1.3lbs – 600 grams – of carrots to get 50 grams of carbohydrate. Highly unlikely. Glycemic load is better for this reason.
